Typical methods for producing an optical fiber utilizing a flame hydrolysis reaction include a vapor phase axial deposition method (hereinafter referred to as "VAD method") and an outside vapor deposition method (hereinafter referred to as "OVD method").
In accordance with the VAD method, fine glass particles produced by the flame hydrolysis reaction are applied onto the tip of a rotating seed rod and grown in its longitudinal direction to produce a porous glass preform (see U.S. Pat. No. 4,135,901). That is, gaseous raw materials of glass (eg. SiCl.sub.4, GeCl.sub.4, POCl.sub.3, BBr.sub.3, etc.) are fed to a burner together with fuel gas and oxygen, and hydrolyzed and oxidized in a flame to form the fine glass particles. The glass particles are deposited as described above to produce a porous preform, which is then sintered at a high temperature to obtain a transparent glass preform. If necessary after molded to reduce its diameter, the transparent preform is drawn to produce an optical fiber.
For example, U.S. Pat. No. 3,737,292 describes the OVD method. Fine glass particles are formed by flame hydrolysis of the raw materials of glass and layers of the fine glass particles are deposited one by one on an outer surface of an elongate starting member in its radial direction. The starting member is then removed to provide a porous preform. The thus formed porous preform is sintered at a high temperature to obtain a transparent preform, which is then, if necessary after molded to reduce its diameter, drawn to produce an optical fiber.
An optical fiber comprises an inner region having a higher refractive index and an outer region having a lower refractive index, and light is transmitted through the high refractive index area. To provide the optical fiber with such two regions, each region is made of silica as a base material and at least one additive which alters, namely, increases or decreases the refractive index of silica. Specific examples of the additive which increases the refractive index of silica are GeO.sub.2, P.sub.2 O.sub.5, Sb.sub.2 O.sub.3 and Al.sub.2 O.sub.3. Specific examples of that decreases the refractive index of silica are B.sub.2 O.sub.3 and F.
Now, the characteristics of the additives is explained by making reference to P.sub.2 O.sub.5 as an additive to increase the refractive index of silica. P.sub.2 O.sub.5 has the following characteristics.
(1) Addition of P.sub.2 O.sub.5 lowers the softening point of silica and thus facilitates its molding.
(2) In the production of the transparent glass preform from the porous preform by the VAD and OVD methods, P.sub.2 O.sub.5 acts as a binder for a material having a higher softening point than P.sub.2 O.sub.5 (eg. SiO.sub.2 and GeO.sub.2) and thus effectively prevents cracking in the porous preform during its production.
(3) Rayleigh scattering in an optical fiber drawn from the transparent preform containing P.sub.2 O.sub.5 is less than in that drawn from the transparent preform not containing P.sub.2 O.sub.5.
Japanese Patent Publication No. 28852/1981 discloses production of a transparent preform containing 2% by weight of P.sub.2 O.sub.5 by the VAD process. Japanese Patent Kokai Publication (unexamined) No. 134128/1979 discloses a transparent preform containing 4% by weight of P.sub.2 O.sub.5. Further, U.S. Pat. No. 4,339,173 discloses a preform containing 3 % by weight or more of P.sub.2 O.sub.5.
In recent years, studies have been made on attenuation through the optical fiber at a wavelength of 1.55 micrometers at which the attenuation is minimum. The attenuation at the 1.55 micrometer band through an optical fiber containing phosphorus is, however, large since P-O-H moieties absorb light having a wavelength of about 1.52 micrometers.
If a porous preform not containing phosphorus is to be sintered, for example, in the VAD the sintering temperature should be considerably high. This undesirably requires a high temperature sintering furnace, a shorter operation life of the furnace, a heater and other facilities, and enlarges the size of the power source and an insulating material that are required.
In order to produce an optical fiber with low attenuation at the 1.55 micrometer band, it is important to reduce the amount of P.sub.2 O.sub.5 contained in the optical fiber. It has now been found that it is desirable for the phosphorus content in the optical fiber to be reduced to 3.times.10.sup.-1 % by mole or less.
It has also been found that distribution of GeO.sub.2 as a dopant to increase the refractive index in the porous glass preform is influenced by the deposition temperature, namely, a surface temperature of the preform, which in turn varies with the concentration of POCl.sub.3 or P.sub.2 O.sub.5 in the flame and, therefore, that the desired distribution of GeO.sub.2 can be achieved by controlling the concentration of a phosphorus compound in the raw materials.
Thus, preferably, the following requirements are to be satisfied:
(1) The porous glass preform prepared by deposition of the fine glass particles contains phosphorus in such an amount that the distribution of a dopant can be easily controlled and further that it can be converted into a transparent preform at a lower sintering temperature than a porous glass preform not containing phosphorus at all; and
(2) The phosphorus content in an optical fiber fabricated by drawing the preform is lower than that in the preform so that the optical fiber has lower attenuation.